1. Field of the Invention
The present invention relates to immersion lithography, and more particularly, to a dynamic fluid control system and method capable of compensating for dynamic changes in the forces exerted on the last optical element and stage by the immersion fluid caused by the motion of the immersion fluid and movements of the stage.
2. Related Art
A typical lithography machine includes a radiation source, an imaging element defining an image pattern, an optical system, and a wafer stage to support and move the wafer. A radiation-sensitive material, such as resist, is coated onto the wafer surface prior to placement onto the wafer table. During operation, radiation energy from the radiation source is used to project the image pattern defined by the imaging element through the optical system onto the wafer. The optical system typically includes a number of lenses. The lens or optical element closest to the wafer is sometimes referred to as the “last” or “final” optical element.
The projection area during an exposure is typically much smaller than the wafer. The wafer therefore has to be moved relative to the optical system to pattern the entire surface. In the semiconductor industry, two types of lithography machines are commonly used. With so-called “step and repeat” machines, the entire image pattern is projected at once in a single exposure onto a target area of the wafer. After the exposure, the wafer is moved or “stepped” in the x and/or y direction and a new target area is exposed. This step and repeat process is performed over and over until the entire wafer surface is exposed. With scanning type lithography machines, the target area is exposed in a continuous or “scanning” motion. The patterning element is moved in one direction while the wafer is moved in either the same or the opposite direction during exposure. The wafer is then moved in the x and y direction to the next scan target area. This process is repeated until all the desired areas on the wafer have been exposed.
Immersion lithography systems use a layer of fluid that fills the gap between the final optical element of the optical assembly and the wafer. The fluid enhances the resolution of the system by enabling exposures with numerical apertures (NA) greater than one, which is the theoretical limit for conventional “dry” lithography. The fluid in the gap permits the exposure with radiation that would otherwise be completely internally reflected at the optical-air interface. With immersion lithography, numerical apertures as high as the index of refraction of the fluid are possible. Immersion also increases the depth of focus for a given NA, which is the tolerable error in the vertical position of the wafer, compared to a conventional lithography system. Immersion lithography thus has the ability to provide resolution down to 50 nanometers or lower.
In immersion systems, the fluid essentially becomes part of the optical system of the lithography tool. The optical properties of the fluid therefore must be carefully controlled. The optical properties of the fluid are influenced by the composition of the fluid, temperature, the absence or presence of gas bubbles, and out-gassing from the resist on the wafer.
The pressure and forces exerted by the immersion fluid on the last optical element and wafer stage should be constant. This desired result, however, is very difficult to achieve for a number of reasons.
With immersion lithography, the fluid is constantly removed and replenished. The removal of the fluid helps recover any contaminants and heat generated during exposure. Ideally, the amount of fluid being supplied should equal the amount being removed. A precise equilibrium, however, is difficult to achieve in practice. An uneven flow rate, which may result in a varying volume of fluid under the last optical element, may cause the forces and pressures acting on the last optical element and wafer stage to be dynamic.
The movement of the wafer stage also creates dynamic forces on the last optical element due to the behavior of the immersion fluid. For example, when the wafer stage starts accelerating, the shape of the fluid at the fluid-air interface, sometimes called the meniscus, changes. The meniscus tends to extend outward at the leading edge and pull-in at the trailing edge of the movement. The change in the shape in the meniscus creates a change in the static pressure exerted on the last optical element and stage by the immersion fluid.
The motion of the stage also creates waves in the immersion fluid. These waves may cause the last optical element to oscillate up and down as well as perturb the wafer stage. If the oscillations are still occurring during an exposure due to the lingering effects of the waves, the accuracy and image quality may be adversely affected.
Vertical adjustments of the wafer may also cause the volume of the gap between the last optical element and the wafer to change. The surface of a wafer is not perfectly flat. Vertical adjustments are made by the wafer stage, depending on the surface topography of the wafer, to maintain the distance between the last optical element and the exposure area constant. The volume of the space between the wafer and last optical element changes when the wafer is moved up and down. As the volume changes, the pressure and forces of the immersion fluid acting on both the last optical element and the wafer stage also change.
The dynamic forces and pressures acting on the last optical element caused by the motion of the immersion fluid may cause the last optical element to become distorted and/or moved either up or down from its ideal position. As a result, the last optical element may be out of focus, resulting in a poor exposure. Similar forces acting on the wafer stage may affect its performance as well.
At high stage speeds the meniscus can be perturbed to the point where it breaks down, particularly at the leading edge. The breakdown is characterized by the escape and deposition of fluid droplets on the wafer where it emerges from the fluid. Such droplets are undesirable. They can entrap air, creating bubbles, when the wafer passes under the immersed lens on a subsequent scan. Also if the droplets dry on the wafer, any contaminants in the droplet, for example residues dissolved from the resist, remain deposited on the wafer.
A dynamic fluid control system and method capable of compensating for dynamic changes in the forces exerted on the last optical element and stage by the immersion fluid caused by the motion of the immersion fluid and movements of the stage is therefore needed.